docs: def to_subject_paths(paths):
      '''
      to_subject_paths(paths) accepts either a string that is a :-separated list of directories or a
       list of directories and yields a list of all the existing directories.
      '''
      if paths is None: return []
      if pimms.is_str(paths): paths = paths.split(':')
      paths = [os.path.expanduser(p) for p in paths]
      return [p for p in paths if os.path.isdir(p)]